Extend chromium_tests trigger to work across masters.

scripts/slave/recipes/chromium.expected/full_chromium_gpu_GPU_Linux_Builder.json

 [
   {
     "cmd": [
       "python",
       "-u",
       "RECIPE_MODULE[depot_tools::bot_update]/resources/bot_update.py",
       "--master",
       "chromium.gpu",
       "--builder",
       "GPU Linux Builder",
@@ skipping 363 matching lines @@
       "--build-properties",
       "{\"blamelist\": [\"cool_dev1337@chromium.org\", \"hax@chromium.org\"], \"build_data_dir\": \"[ROOT]/build_data_dir\", \"buildername\": \"GPU Linux Builder\", \"buildnumber\": 571, \"mastername\": \"chromium.gpu\", \"parent_buildername\": null, \"recipe\": \"chromium\", \"slavename\": \"TestSlavename\", \"workdir\": \"/path/to/workdir/TestSlavename\"}"
     ],
     "name": "package build"
   },
   {
     "cmd": [],
     "name": "trigger",
     "trigger_specs": [
       {
+        "bucket": "master.chromium.gpu",
         "builder_name": "Linux Release (NVIDIA)",
         "properties": {
           "parent_got_angle_revision": "fac9503c46405f77757b9a728eb85b8d7bc6080c",
           "parent_got_angle_revision_cp": "refs/heads/master@{#297276}",
           "parent_got_nacl_revision": "d998e125e8253980d76e269b7982aeeefc1b9b50",
           "parent_got_nacl_revision_cp": "refs/heads/master@{#274981}",
           "parent_got_revision": "f27fede2220bcd326aee3e86ddfd4ebd0fe58cb9",
           "parent_got_revision_cp": "refs/heads/master@{#170242}",
           "parent_got_swarming_client_revision": "a0ee5d99f1d4b38ddaa4c759c837980b63b99566",
           "parent_got_swarming_client_revision_cp": "refs/heads/master@{#276089}",
           "parent_got_v8_revision": "801ada225ddc271c132c3a35f03975671d43e399",
           "parent_got_v8_revision_cp": "refs/heads/master@{#43426}",
           "parent_got_webrtc_revision": "0f90628433546e61d9268596da93418c623137f4",
           "parent_got_webrtc_revision_cp": "refs/heads/master@{#120644}"
         }
       }
     ],
     "~followup_annotations": [
       "@@@STEP_TEXT@Linux Release (NVIDIA)@@@"
     ]
   },
   {
     "name": "$result",
     "recipe_result": null,
     "status_code": 0
   }
 ]